In integrated photonics, there is an ongoing effort to design structures which efficiently couple light among nanoscale waveguides and standard single-mode optic fibers. The waveguide grating coupler is one such structure which employs a Bragg grating to diffract light out of the plane of a waveguide so that it may be received by a fiber. One problem faced by grating couplers currently is that, at the interface between the waveguide and the grating, a portion of the incident light gets reflected back into the waveguide and propagates counter to the initially injected mode. This portion of the light, in addition to not being coupled into the output fiber, can pollute the signal at other of the chip's output ports.
In some prior approaches, unwanted back-reflections have been inhibited by orienting the front face of the grating coupler at an oblique angle to the incident beam. This technique does not, however, convert the reflected power into light which diffracts from the grating as intended, and so does not improve the diffraction efficiency.